In recent years, with the development of magnetic recording systems, there is a demand for higher image quality and sound quality, particularly for magnetic tapes for video use. In order to meet these needs, the particle size of ferromagnetic powders has been decreased, and the density of magnetic recording media has been increased.
Furthermore, with consumption of magnetic recording media in large quantities, it is desirable to produce the recording media at lower cost. One means for reducing the production cost of magnetic recording media is to provide a plurality of magnetic layers. In this case, an upper magnetic layer is designed to provide good image quality, and a lower magnetic layer is designed to provide good sound quality. Since the above noted functions can thus be separated into the upper and lower layers, a ferromagnetic powder also favorable from a cost aspect can be used. Also, appropriate materials other than the ferromagnetic powder can be suitably selected to minimize production cost.
On the other hand, in order to achieve high image quality and sound quality for a magnetic tape for video use, an increased reproduced output and reduced modulation noise is required to improve S/N. For this purpose, appropriate selection of the electromagnetic characteristics of the ferromagnetic powder is important. For example, regarding the above-described magnetic recording media having a plurality of magnetic layers, it is necessary to increase the loading of the ferromagnetic powder and to reduce the particle size of the ferromagnetic powder to provide an improvement in output and a reduction in noise in the upper magnetic layer for good image quality. Furthermore, in the lower magnetic layer, it is necessary to optimize the coercive force in consideration of linear audio characteristics and also video characteristics, particularly with respect to maintaining chroma output.
However, conventional plural-layer magnetic recording media for video use have a low residual magnetic flux density of the upper magnetic layers, such that the video output and S/N are insufficient. Furthermore, the coercive force of the upper magnetic layer is not necessarily optimally matched to that of the lower magnetic layer. For this reason, the chroma output and S/N and linear audio frequency characteristics are not sufficient.
On the other hand, with respect to the ferromagnetic powder used in the magnetic layers, ferromagnetic metal powders, barium ferrite and the like have excellent magnetic characteristics, and are advantageous for high density recording. However, these ferromagnetic powders are expensive and have poor storage stability. Furthermore, it is difficult to disperse these ferromagnetic powders in a binder such that their potentially good performance is not always realized. Ferromagnetic oxide powders have few problems in this regard and therefore practically advantageous. Investigators have sought to further improve the characteristics of ferromagnetic oxide powders.
In particular, of the ferromagnetic iron oxide powders, cobalt-containing iron oxide powders exhibit excellent magnetic characteristics, and iron oxide powders having various characteristics are proposed (for example, in JP-B-48-39639 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-51-36596 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-51-38079 and JP-A-51-16905).
Magnetic recording media utilizing the features of a plural magnetic layer structure and using cobalt-containing iron oxide as the ferromagnetic powder are proposed, for example, in JP-A-1-220218, wherein cobalt-modified iron oxide having a mean length in the long axis direction of 0.27 to 0.35 .mu.m, an Fe.sup.2+ /Fe.sup.3+ mole ratio of 1/100 to 2.6/100 and a coercive force of 360 to 440 Oe is used in the lower layer, and cobalt-modified iron oxide having a mean length in the long axis direction of 0.17 to 0.27 .mu.m, an Fe.sup.2+ /Fe.sup.3+ mole ratio of 5.0/100 to 6.5/100 and a coercive force of 550 to 720 Oe is used in the upper layer.
In the invention disclosed in JP-A-1-220218, the ferromagnetic powder contained in the lower layer is designed to smooth the magnetic layer by decreasing the particle size and to improve print-through characteristics by reducing the proportion of Fe.sup.2+, and the ferromagnetic powder contained in the upper layer is designed to heighten the output through a reduction in noise by further decreasing the particle size and to improve susceptibility by increasing the proportion of Fe.sup.2+. However, the invention disclosed in JP-A-220218 concerns a magnetic tape for audio use. This tape is still inadequate for a video tape of the VHS system or the .beta. system in terms of the coercive force of the ferromagnetic powder and the particle size.
Namely, when the magnetic recording medium disclosed in JP-A-220218 is used for a video application, the video output and S/N are decreased because the coercive force of the ferromagnetic powder contained in the upper layer is as low as 550 to 720 Oe, the mean length in the long axis direction is as large as 0.17 to 0.27 .mu.m and the Fe.sup.2+ /Fe.sup.3+ mole ratio is as low as 5.0/100 to 6.5/100. Furthermore, the linear audio frequency characteristics are lower than the VHS standard, because the coercive force of the ferromagnetic powder contained in the lower layer is as low as 360 to 440 Oe.